DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

BARE-BONES SPECTRUM ANALYZER 161magnetic heads from a discarded drum. Even a very worn-out head will work well in thisapplication. Very soiled heads should be cleaned with a swab and pure alcohol. Degaussingwill also help improve the sensitivity of an old head. All other aspects of constructing andusing this probe are the same as for the ferrite-bead probe.For E-fields, the simplest near-field probe is a coax cable in which a short segment ofthe center conductor extends beyond the braid at the unterminated end of the coax. Similarto the loop probe, a longer wire will pick up a stronger signal at the expense of specificityand bandwidth. In general, the length of the wire should be selected so that measurementscan be performed with a sensitivity of approximately 3 mV/m. At this level, potentiallyproblematic emissions can be identified without causing undue concern about low-levelemissions.Constructing the ideal H- or E-field probe for a specific job may take some trial anderror, since the effort of electromagnetic modeling required for proper design is most probablyan overkill for most applications. One test that you may nevertheless want to performon a probe is to determine the existence of resonances within the desired spectral range.To conduct the test, a wideband probe should be connected to an RF generator set up totrack the tuning frequency of a wideband spectrum analyzer. The probe under test shouldbe located in close proximity to the emitting probe and connected to the input of the spectrumanalyzer. The limit of the useful bandwidth of a probe is the point at which the firstabrupt resonance appears.Before even plugging the spectrum analyzer to the power line, however, the first step inconducting a near-field EMI study should be to draw a component placement diagram ofthe assembly to be probed. The diagram should indicate circuit points identified in themathematical circuit harmonic analysis as potential sources for EMI radiation. Only afterthis preliminary work has been done should bench testing begin. A coarse near-field sweepshould be conducted at relatively high gain to identify EMI hot spots in the assembly. Atechnique that works well is to log the frequencies at which strong components appearwhen scanning the unit under test. Detailed scanning using a more discriminating probecan then concentrate on the hot spots to identify the culprit circuit generating offendingemissions.A very valuable source of clues for future troubleshooting can be built along the way byprinting the spectral estimate at each point in which measurements strongly agree or disagreewith the circuit’s harmonic analysis. In any case, keep detailed and organized notesof the near-field scans, since these will certainly prove to be invaluable when attemptingquick fixes while the clock is running at the far-field compliance-testing facility.BARE-BONES SPECTRUM ANALYZERWhile an ac voltmeter can provide an indication of the field strength to which a probe isexposed, it does not provide any indication of the spectral contents of an emission. A spectrumanalyzer is a tool that certainly cannot be beaten in the search for offending signals.Unfortunately, spectrum analyzers are often beyond the reach of a designer on a tight budget.For near-field sniffing, however, even the crudest spectrum analyzer will do a magnificentjob.Figure 4.9 shows a simple home-brewed adapter to convert any triggered oscilloscopeinto a spectrum analyzer capable of providing qualitative spectral estimates with a bandwidthof 100 kHz to 400 MHz. As shown in Figure 4.10, a voltage-controlled TV tuner IC1forms the basis of the spectrum analyzer. Most any voltage-controlled tuner will work, andyou may be able to get one free from a discarded TV or VCR printed circuit board. The connectionpoints and distribution vary from device to device, but the pinout is usuallyidentified by stampings on the metallic can of the device.